I applied online. The process took 2 weeks. I interviewed at US Marine Corps (Quantico, VA) in Feb 2011
Interview
The first contact is made by the recruiter who screens the applicant for suitability to the position. Next phone interviews were conducted by two hiring managers. Unfortunately they were both not available on the same day so I had to do a screening interview with each at different times.
The next step was a face to face interview with both hiring managers and their business partners where they each asked questions from a list of approximately 100 questions.
The process took 5 months. I interviewed at US Marine Corps in Jan 2008
Interview
If you have a +1000 SAT, + 3.0 GPA, no medical problems, good athletics, no criminal history and have a normal, well adjusted disposition, you have a good shot at making it into OCS.
Find a recruiter that will work with you and make sure you start taking care of all of the stuff you need to do.
Once you get to OCS remember that their goal is to weed people out and your goal is to stay in, so don't give up. The bar of gold at the end of the rainbow is worth it.